In general, raster display systems operate by storing character or image data representing at least one image frame in a memory and displaying text or other images on a cathode ray tube or the like by accessing the memory. Recently, it has been proposed to produce synthesized images by combining different image data to create a frame of data.
An example of such an arrangement is shown in Japanese Published Patent Application No. 185085/82 entitled "Image Display". In this system a prohibition color is specified before display data is written into an image memory from, for example, a floppy disk drive. During the write operation, previous display data is left at locations where the prohibition color is assigned and the new display data is written into the other locations. This allows the composition of a synthesized image comprising an image formed from a plurality of input images. For example, if an image of a bus, as shown in FIG. 6, is stored in an image memory, and the body color, red, and tire color, black, are defined as prohibition colors, then landscape data, as shown in FIG. 5, can be written into the image memory without overwriting the bus image. The combined image can now be read for display on a cathode ray tube display device. With this system, a problem arises when complex image processing, such as the production of animated images, is attempted. If, for example, it is required to move the bus in FIG. 6 across the screen with a fixed background, then the background image data needs to be processed continuously. This requires a complex program and lowers the processing speed.
Other examples of system in which images are built from different portions are shown in Japanese Published Patent Applications No. 161839/79 entitled "Image Generation" and 167079/82 entitled "Overwrite Control System for Graphic CRT".
In the first of these, an image is generated by combining a plurality of basic geometric figures. These figures are defined by parameters, some of which are given a transparency attribute. With such an attribute, the background in the final image can appear through that figure. Thus, movement of that Figure allows corresponding areas of the background to appear without the need for complex programming. However, as the combined image is formed only from basic geometric figures, this arrangement is highly restricted. It cannot, for example, form the images shown in FIGS. 5-8.
In the second of these applications, a technique is disclosed for displaying a plant process which continuously changes. The display data is broken down into a number of elements, each of which is stored in a separate frame memory. In each frame memory, predetermined data is written in locations corresponding to the associated element with the remaining locations being defined as non-data areas. The frame memories are given a priority order and to provide a display, each corresponding location in each frame memory is tested in turn in an order defined by the priorities. In testing, non-data areas are ignored and the element data is applied for display on a CRT. This listing and application process is performed for all the locations in the frame memories in synchronism with the CRT scanning.
This system is convenient when each element is moved and displayed as there is no need to take account of the background. However, it is difficult to display images hidden one by another or to make an image of an element transparent.